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Physics 101: Lecture 14, Pg 3 Concept Quiz The picture below shows three different ways of using a wrench to loosen a stuck nut. Assume the applied force F is the same in each case. In which of the cases is the torque on the nut the biggest? A. Case 1 B. Case 2 C. Case 3 CORRECT 10

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Physics 101: Lecture 14, Pg 4 Concept Quiz The picture below shows three different ways of using a wrench to loosen a stuck nut. Assume the applied force F is the same in each case. In which of the cases is the torque on the nut the smallest? A. Case 1 B. Case 2 C. Case 3 CORRECT 12

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Physics 101: Lecture 14, Pg 11 Equilibrium Quiz and Example The picture below shows two people lifting a heavy log. Which of the two people is supporting the greatest weight? 1. The person on the left is supporting the greatest weight 2. The person on the right is supporting the greatest weight 3. They are supporting the same weight CORRECT 34 mg FLFL FRFR L/2 L Look at torque about center: F R L – F L L/2 = 0 F R = ½ F L

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Physics 101: Lecture 14, Pg 16 Torque and rotational motion Torque is the quantity that changes rotational motion. For rotations, it is the object that replaces force as the cause of angular acceleration.

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Physics 101: Lecture 14, Pg 18 Moment of inertia I =  i m i r i 2 Net torque leads to angular acceleration, but we need the moment of inertia to tell us how large an acceleration results. Moment of inertia in rotational problems takes the place that mass has in translational problems. Depends upon the mass and location of the mass from the axis of rotation. Large moment of inertia requires large torque to accelerate object (just like large mass requires large force)

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Physics 101: Lecture 14, Pg 19 Consider a rod of uniform density with an axis of rotation through its center and an identical rod with the axis of rotation through one end. Which has the larger moment of inertia? A IC > IE B IC < IE C IC = IE Concept Quiz

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Physics 101: Lecture 14, Pg 20 Consider two masses, each of size 2m at the ends of a light rod of length L with the axis of rotation through the center of the rod. The rod is doubled in length and the masses are halved. What happens to I? A: I A > I B B: I A < I B C: I A = I B Concept Quiz

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Physics 101: Lecture 14, Pg 21 Consider a solid disk with an axis of rotation through the center (perpendicular to the diagram). Two holes are cut out near the center and the material is placed near the rim. Which has the larger I? A I A > I B B I A < I B C I A = I B Concept Test

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Physics 101: Lecture 14, Pg 22 A mass m hangs from string wrapped around a pulley of radius R. The pulley has a moment of inertia I and its pivot is frictionless. Because of gravity the mass falls and the pulley rotates. The magnitude of the torque on the pulley is.. A: greater than mgR B: equal to mgR C: less than mgR (Hint: Is the tension in the string = mg?) Concept Test